1. Field of the Invention
The invention relates generally to switchgear apparatus for electrical power distribution, and more particularly to a modular bucket assembly adapted for insertion into compartments of arc resistant switchgear. The bucket assembly retains electrical distribution and control apparatus, such as for example circuit breakers and instrumentation. A complete bucket assembly, including retained electrical apparatus, is adapted for insertion into a switchgear cabinet compartment. The present invention modular bucket assembly allows a switchgear manufacturer to construct arc resistant switchgear selectively to different service personnel accessibility levels. The modular bucket assembly of the present invention also facilitates subsequent installation and upgrade at field sites or overhaul service centers, should the switchgear operator wish to modify the switchgear accessibility level after manufacture.
2. Description of the Prior Art
Switchgear and switchboard are general terms which cover metal enclosures housing switching and interrupting devices, such as fuses and circuit breakers, along with associated control, instrumentation and metering devices; also assemblies of these devices with associated buses, interconnections and supporting structures used for distribution of electric power. Low voltage switchgear and switchboards operate at voltages up to 635 volts and with continuous currents that can exceed 5000 amperes. They are designed to withstand short-circuit currents up to 200,000 amperes.
Typical low voltage switchgear equipment is often an assembly composed of multiple metal enclosed sections or cabinets. Each section may have several circuit breakers stacked one above the other vertically in the front of the section with each breaker being enclosed in its own metal compartment. Each section has a vertical or section bus which supplies current to the breakers within the section cabinet via short horizontal branch buses that extend through insulated openings in the rear wall of the breaker compartments. The vertical buses in each section are supplied with current by a horizontal main bus that runs through the line-up. The rear of the section is typically an open area for the routing of cables.
Low voltage switchgear and switchboards are typically designed to withstand the effects of bolted (non-arcing) faults on the load terminals and this capability is validated during Short-Circuit Current and Short-Time Current Withstand Tests in Institute for Electrical and Electronics Engineers (“IEEE”) Standard C37.20.1, the disclosure of which is incorporated herein by reference.
Standard metal-enclosed switchgears are designed to withstand the mechanical forces generated by bolted faults on the load terminals until a power circuit breaker or other protective device can interrupt the flow of fault current. This capability is verified by short-circuit and short-time withstand tests on the equipment and interruption tests on the power circuit breakers. During a bolted fault, the voltage at the fault location is essentially zero and the fault energy is dissipated throughout the power system. The arc generated within the power circuit breaker during interruption is cooled and extinguished by the circuit breaker internal arc chutes. The minimal out gassing of arc byproducts from the arc chutes is contained by the switchgear, as verified by interruption tests.
The occurrence of an arcing fault inside the switchgear produces physical phenomena that are different from bolted faults. An internal arcing fault can be caused by insulation degradation, insulation contamination, entrance of vermin, foreign objects coming into contact with the energized bus, or any other unplanned condition that creates an electrical discharge path through air. During an arcing fault, the voltage at the fault location is essentially the system voltage and the fault energy is focused within the switchgear enclosure. Arc temperatures can exceed 20,000 degrees Kelvin, rapidly heating the air and vaporizing metal parts. The expanding plasma creates mechanical and thermal stress in the equipment that potentially can be released outside the switchgear cabinet. Materials involved in or exposed to the arc produce hot decomposition products, both gaseous and particulate, which may be discharged to the outside of the enclosure. In the event of an arcing fault in switchgear, is desirable to vent hot decomposition products in directions where they are not likely to contact personnel.
One known way to avoid personnel contact with decomposition products released by switchgear undergoing an arc fault is to prohibit personnel from being present when the switchgear is energized. This is not practical when it is required to service one or more switchgear compartments while other compartments must remain energized to perform necessary electrical distribution and control functions.
Thus a need exists in the art for arc resistant switchgear that directs hot decomposition products created by an internal arcing fault away from the front, sides and rear of switchgear equipment while personnel must be present to service portions of the energized switchgear. Commonly owned U.S. Pat. No. 7,821,774 entitled. “Mounting Base With Arc Plenum” and pending U.S. patent application Ser. No. 12/362, 715, filed Jan. 30, 2009 and claiming priority of Provisional Application No. 61/030,321, filed on Feb. 21, 2008, the complete contents of both of which are fully incorporated by reference herein, are directed to inventions for channeling internal hot arc gasses away from the sides of switchgear.
Another need exists in the art for arc resistant switchgear having enclosure compartments capable of selectively meeting different personnel accessibility standards while being energized: for example a single switchgear section cabinet that accommodates both circuit breaker or other electrical apparatus compartments along with adjoining instrumentation and control compartments. During switchgear energization the circuit breaker compartments coupled to the full “live” power voltage and amperage. Accordingly, circuit breaker compartment doors are kept closed while the switchgear is energized. Instrumentation and control component compartments are not coupled to the full “live” power, and are generally operated with relatively low AC power of under 120 volt and/or DC power of 24 volt or less. Instrumentation compartment doors may need to be opened by personnel during switchgear energization, so that they can view and record instrument readings or adjust equipment settings.
The need to combine both instrumentation and circuit breaker compartments in the same switchgear section cabinet creates another need for compartment configurations capable of meeting all personnel accessibility standards that can be readily configured and reconfigured during manufacture, subsequent retrofitting or repair in response to changing needs of the switchgear operator.